The invention relates to multi-section integrated semiconducting devices or lasers, comprising resonator sections, being either distributed reflection or transmission sections. The invention also relates to methods for widely wavelength tuning semiconductor devices or lasers.
Tuning of a conventional Distributed Bragg Reflector (DBR) semiconductor laser is limited by the fact that the relative tuning range is restricted to the relative change in the refractive index of the tuning region. This means that the tuning range, under normal operating conditions, cannot exceed 10 nm. This is substantially less than the potential bandwidth, restricted by the width of the gain curve, which is about 100 nm. Such conventional DBR lasers can functionally be characterized as comprising a first part being a two-sided active section, for creating radiation, for instance a light beam, by spontaneous emission over a bandwidth around one center frequency. Said first part also guides said radiation or light beam. Such conventional DBR lasers further have two reflectors. Said reflectors are bounding said two-sided active section, thus one at each side.
The limited selectivity problem has been recognized by Wolf, et al (European Transactions on Telecommunications and Related Technologies, 4 (1993), No. 6) showing in FIG. 10 a laser structure with two parallel waveguides but without gratings. These two parallel waveguides cannot be considered as resonators, indeed the spectra (shown in FIG. 10b and c) show the comb mode spectra corresponding to arms B and A, but they are either the comb mode spectra of arm B, the gain section and the reflectors R or the comb mode spectra of arm A, the gain section and the reflectors R. As the spacing between the spectral lines are determined by the length of the structures, it appears that said spacing is still very small, resulting in still a low selectivity and a low tuneability.
Over the past years several advanced laser structures have been proposed with an extended tuning range. Examples are the Y-laser [M. Kuznetsov, P. Verlangieri, A. G. Dentai, C. H. Joyner, and C. A. Burrus, xe2x80x9cDesign of widely tunable semiconductor three-branch lasers,xe2x80x9d J. Lightwave Technol., vol. 12, no. 12, pp. 2100-2106, 1994], the co-directionally coupled twin-guide laser [M.-C. Amann, and S. Illek, xe2x80x9cTunable laser diodes utilizing transverse tuning scheme,xe2x80x9d J. Lightwave Technol., vol. 11, no. 7, pp. 1168-1182, 1993], the Sampled Grating (SG) DBR laser [V. Jayaraman, Z. M. Chuang, and L. A. Coldren, xe2x80x9cTheory, design and performance of extended tuning range semiconductor lasers with sampled gratings,xe2x80x9d IEEE J. Quantum Electron., vol. 29, no. 6, pp. 1824-1834, 1993], the Super Structure Grating (SSG) DBR laser [H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, xe2x80x9cQuasicontinuous wavelength tuning in super-structure-grating (SSG) DBR lasers,xe2x80x9d IEEE J. Quantum Electron., vol. 32, no. 3, pp. 433-440, 1996] and the Grating assisted Coupler with rear Sampled Reflector (GCSR) laser [M. xc3x96berg, S. Nilsson, K. Streubel, L. Bxc3xa4ckbom, and T. Klinga, xe2x80x9c74 nm wavelength tuning range of an InGaAsP/InP vertical grating assisted codirectional coupler laser with rear sampled grating reflector,xe2x80x9d IEEE Photon. Technol. Lett., vol. 5, no. 7, pp. 735-738, 1993]. In the first two types of devices, a trade-off had to be made between the tuning range and the spectral purity (broad tuning range vs. high Side Mode Suppression Ratio (SMSR)). Therefore recently most research attention has gone to the (S)SG-DBR and GCSR lasers.
A sampled grating DBR laser, comprises of two sampled gratings exhibiting a comb-shaped reflectance spectrum, with slightly different peak spacing due to the different sampling periods. As an alternative, other grating shapes can be used: these are normally referred to as xe2x80x9csuper structure gratingsxe2x80x9d (SSG). Lasers of this type have been fabricated with tuning ranges up to about 100 nm. The operation of the device is such that through current injection in the two DBR sections, a peak of the front and rear reflectance comb are aligned at the desired wavelength. The phase section is used to align a longitudinal cavity mode with the peaks of the two reflectors. The disadvantage of the (S)SG-DBR approach is that light coupled out of the laser has to pass a long passive or inactive section, leading to loss. Also, the losses in the two reflector sections increase with the amount of current injected into those sections, leading to a tuning current dependent output power.
The SG-DBR laser and the SSG-DBR laser are functionally characterized as comprising a two-sided active region for light creation and two reflectors one at each side of the active region, said reflectors having a reflection characteristic with a plurality of reflection peaks. Said characteristic has spaced reflection maxima points providing a maximum reflection of an associated wavelength. Such a characteristic can be obtained via sampled gratings, which exhibit a comb-shaped reflection spectrum or via the so-called supergratings. Said gratings or supergratings can also be characterized as distributed reflectors.
Sampled gratings can be described as structures in a waveguide system, having a periodically broken short-period structure including short period stripped regions alternating with non-stripped regions. The supergratings can be described as structures in a waveguide system having a diffractive grating having a plurality of repeating unit regions, each having a constant length, thus forming a modulation period, and at least one parameter that determines the optical reflectivity of said diffractive grating varying depending on its position in each of said repeating unit regions along a direction of optical transmission in said laser, said diffractive grating extending by at least two modulation periods. Reference is made to U.S. Pat. No. 5,325,392 related to distributed reflector and wavelength tunable semiconductor lasers, which is hereby incorporated by reference in its entirety.
The SG-DBR laser and the SSG-DBR laser exploit constructive interference of the periodic characteristics of reflectors, located at different sides of the active section, with different periodicity, to obtain a wide tunability. The alignment of the reflector peaks can be described by stating that the spacing of said reflective maxima points of the reflectors are different or are essentially not equal and only one said reflective maxima of each of said reflectors is in correspondence with a wavelength of said created lightbeam. Reference is made to patent U.S. Pat. No. 4,896,325, related to multi-section tunable lasers with differing multi-element mirrors, which is hereby incorporated by reference in its entirety.
As the construction of said reflectors leads to long inactive sections, this results in lasing output power losses.
Other lasers, which use a co-directional coupler, readily have a very wide tuning range, but there is insufficient suppression of neighbouring longitudinal modes. The combination of a widely tuneable but poorly selective co-directional coupler with a single (S)SG reflector will give both wide tuning and a good side mode suppression. Furthermore, the optical output signal does not pass through a passive region. Again tuning of 100 nm has been achieved. Unfortunately, such a structure is rather complicated to manufacture, requiring at least 5 growth steps. Reference is made to patent U.S. Pat. No. 5,621,828 related to integrated tunable filters, which is hereby incorporated by reference in its entirety.
EP-A-0926787 describes a series of strongly complex coupled DFB lasers. In the disclosed structure, gratings are made within the active sections. Said gratings are selected such that no substantial interaction between the lasers, defined by a grated active section, in series is obtained. The disclosed structure enables generation of multiple wavelengths, even sumultaneously, but does not address the issue of selectivity and tuneability.
A parallel structure with a plurality of waveguides is disclosed in the PATENT ABSTRACT OF JAPAN, vol. 013, no. 026, Jan. 20, 1989, JP 63 229796 (Fujitsu Ltd. The disclosed structure again enables radiation of a plurality of wavelengths but does not address the issue of tuneability. The optical switch is operated for selecting a waveguide, thus no simultaneously optical connection between said waveguide is obtained.
The aim of the present invention is to disclose laser structures which are easy to manufacture and which are widely tuneable and have low lasing output power losses.
In the present invention, alternative laser structures, apparatus or devices are presented, which have potentially the same tuning performance as (S)SG-DBR and GCSR lasers and which output power, and not pass through a long passive region.
An integrated/semiconductor tunable laser comprising a substrate made of a semiconducting material, a two-sided active section on said substrate, and a plurality of sections on said substrate, is disclosed. Said laser can be denoted as a multi-section integrated semiconductor laser. Said active section is radiation generating, for instance, but not limited to the range of optical radiation. All said sections are connected to one side of said active section. Note that this does not mean that they are directly coupled to said active section. In case of optical radiation, said connection can be denoted as an optical connection. At least two of said sections include a waveguide system. Each of said sections defines a resonator.
These resonator sections have a spectra with spaced maxima resonant points themselves. They are themselves either a filter or a reflector with a comb mode spectra.
The resonators used in the present invention have resonant characteristics with a plurality of resonant peaks. Alternatively it can be said that said resonators have spaced resonant maxima points providing a maximum resonance of an associated wavelength. The transmission filters used in the present invention have a transmission characteristic with a plurality of transmission peaks. Alternatively it can be said that said transmission filters have spaced transmission maxima points providing a maximum transmission of an associated wavelength. The reflectors used in the present invention have a reflection characteristic with a plurality of reflection peaks. Alternatively it can be said that said reflectors have spaced reflective maxima points providing a maximum reflection of an associated wavelength.
The spacing of said resonator maxima points corresponding to the transmission or reflective maxima points of at least two of said sections are selected to be essentially not equal or different. Said laser is therefore denoted as an integrated semiconductor laser with different reflection or transmission sections. The transmission and reflection characteristic of said transmission filters and reflectors are positioned relative to each other such that at least one of said transmission or reflective maxima of each of said two sections overlap each other. This means that said sections have at least one transmission or reflective maxima for a same first frequency. Due to the different spacings of said transmission or reflective maxima points a small shift of one of said transmission or reflective characteristics can result in overlapping of at least one other transmission or reflective maxima of each of said two sections. Said sections have then at least one second frequency in common, which can be largely different from said first frequency. Said shift can be due to current injections in said transmission or reflective sections. It can be said that said laser comprises means for injecting current into some of said plurality of sections, resulting in said transmission or reflection characteristic being shifted in wavelength. Said overlapping maxima points define a plurality of lasing wavelengths. Due to the small shifting of at least one resonator characteristic, said device jumps from a first set of lasing wavelengths to another set of lasing wavelengths. The spacings of the maxima resonant points are accordingly essentially determined by the grating instead of the length of the sections.
It can be said that said active section creates a radiation or a lightbeam by emission and that the device emits an emitted laser beam with the wavelength of said emitted lightbeam being in correspondence with said overlapping maxima of said transmission filters or reflectors. Said active section is thus creating radiation or a light beam by spontaneous emission over a bandwidth around a center frequency and guides said radiation or light beam and has (optical) amplification actions. Said emitted radiation or lightbeam does not pass through said plurality of sections. The combination of said plurality of sections, having a combinated reflection action, and said (optical) amplification action of said active section causes lasing at said set of lasing wavelengths. Due to the fact that small shifting of resonator characteristics results in large difference in the set of lasing wavelenghts, an optical laser having a wide tunability, is obtained. Said laser is therefore denoted widely wavelength tunable integrated semiconductor laser.
According to a preferred embodiment of the present invention, the active radiation-generating section is connected at one side to a plurality of grated sections, but said gratings are not included in said active section. Moreover the gratings are selected such that substantial interaction between the spectra of said gratings can be used because this is the working principle used for improving the selectivity. Therefore at least one of said resonant maxima of each of two said sections are overlapping with each other.
In an embodiment of the invention, only one of said resonator maxima points is overlapping. It can then be said that said active section creates a radiation or a lightbeam by emission and the device emits an emitted laser beam with the wavelength of said emitted lightbeam being in correspondence with said overlapping maxima of said transmission filters or reflectors. The combination of said plurality of sections, having a combinated reflection action with a single reflection wavelength, and said (optical) amplification action of said active section causes lasing at said single reflection wavelength, defined by said overlapping resonator maxima points.
In an embodiment of the invention, at least one of said plurality of sections is inactive. This means that such inactive section is not creating a lightbeam by emission.
In an embodiment of the invention, at least one of said plurality of sections is active. This means that such active section also creates a lightbeam by emission.
In an embodiment of the invention, at least one of said waveguide systems has a periodically broken short-period structure including short period stripped regions alternating with non-stripped regions. Such waveguide systems are also denoted as distributed, hence said laser is denoted a semiconductor laser with distributed reflection or transmission sections. In one aspect of the invention, two such waveguides can be found on the same side of the active region.
In an embodiment of the invention, at least one of said waveguide systems has a diffractive grating having a plurality of repeating unit regions each having a constant length, thus forming a modulation period, and at least one parameter that determines the optical reflectivity or transmission of said diffractive grating varying depending on its position in each of said repeating unit regions along a direction of optical transmission in said laser, said diffractive grating extending by at least two modulation periods.
In an embodiment of the invention, at least one of said waveguide systems is a ring resonator.
In an embodiment of the invention, the laser further comprises a plurality of power splitters, being exploited for optically connecting part of said plurality of sections and connecting part of said plurality of sections with said active section.
In an embodiment of the invention, said laser is a serial concatenation of said active section and a plurality of said sections.
In an embodiment of the invention, said laser is a connection of said active section to one single port side of a power splitter and a parallel connection of a plurality of sections to the other multi port side of said power splitter.
In an embodiment of the invention, said laser comprises phase sections, being exploiting for adjusting the round trip cavity phase and thus a lasing mode wavelength of the laser.